In a so called thermal transfer printing apparatus, the printhead includes a plurality of thermal heating elements which are selectively energisable by a controller during printing to warm and soften pixels of ink from the tape and to transfer such pixels to the substrate. The printhead presses the tape against the substrate such that the pixels of ink contact the substrate before the web of the tape is peeled away, thus transferring the pixels of ink from the tape to the substrate.
A thermal transfer overprinter is used to print on to a product's primary packaging and typically mounts within a packaging machine. The image to be printed is often a date code or other product information which needs to be applied to the product's packaging as close as possible to the time at which the product was packaged. The tape drive is used to move and position the thermal transfer tape.
In order to avoid wasting ink, whilst maintaining acceptable print quality, it is advantageous to be able accurately to control the movement of the tape, so as to position the next portion of tape to be used directly adjacent a portion of the tape from which the ink has previously been removed. It is desirable for a spacing between adjacent regions of tape from which pixels are removed to create an image, to be less than 1 mm.
It is also important to ensure that the regions of tape from which ink is removed during successive printing operations do not overlap, so that the printhead does not attempt to remove ink from a region of the tape from which the ink has already been removed. However, it is known to interlace images, such that a previously used region of tape is reused, but in the second and/or subsequent printing operations, different pixels of ink are removed from the tape to create an image.
It is known to provide thermal transfer printing apparatus in two different configurations. In the first, so called “intermittent” configuration, the substrate to be printed and the tape are held stationary during a printing operation, whilst the printhead is moved across the area of the substrate to be printed. Once the printing operation is complete, the printhead is lifted away from the tape, and the tape is advanced to present a fresh region of tape to the printhead for the next printing operation.
In the second, so called “continuous” configuration, the substrate to be printed moves substantially continuously and the tape is accelerated to match the speed of the tape before the printhead is brought into thermal contact with the tape and the printing operation is carried out. In this configuration, the printhead is maintained generally stationary during each printing operation.
In the case of a printing apparatus in continuous configuration, it is also necessary to accurately control the speed of the tape, to ensure that it matches the speed of the substrate. A typical thermal transfer printer operates with substrate that advances at linear speeds between approximately 0.01 meters per second and approximately 2 meters per second. Typical substrate accelerations are up to approximately 12 meters per second per second.
Printing apparatus of the kind described above includes drive apparatus for moving the tape relative to the printhead, to present fresh tape, from which pixels of ink are yet to be removed, to the printhead, such that successive printing operations can be carried out. It has long been known to provide tape drives which include two spool supports, one of which supports a supply spool on which unused tape is initially wound, and the other of which supports a take-up spool, onto which the tape is wound after it has been used. Tape extends between the spools in a tape path. Each of the spool supports, and hence each of the spools of tape, is drivable by a respective motor.
The tape used in thermal transfer printers is thin. Therefore it is important to ensure that the tension in the tape extending between the two spools is maintained at a suitable value or within a suitable range of tensions, in particular to enable the web to peel cleanly away from the heated ink. Too much tension in the tape is likely to lead to the tape being deformed or broken, whilst too little tension will inhibit the correct operation of the device. A slack tape is likely to affect print quality.
It is known to provide various types of tape drive which are compatible with thermal transfer overprinters. For example, it is known to provide a pair of stepper motors, each of which controls the movement of one of the spools so as to advance tape between the spools in a desired direction. The stepper motors are driven in a co-ordinated manner to transfer the tape from the supply spool to the take up spool and to accurately position the tape adjacent the printhead, whilst maintaining tension in the tape within an acceptable range. Various methods of determining and maintaining tension in the tape are known. Such methods require the tension in the tape to be measured, and for a correction to be applied in the event that the tension has strayed or is straying beyond a limit of the acceptable range. Therefore, such methods incur a delay of at least one printing operation between the tension in the tape falling outside the acceptable range and an appropriate correction being applied. It may even be the case that only a partial correction can be applied during the next printing operation, and thus the delay in correcting the tension in the tape such that it falls within the acceptable range is more than one printing operation.
It is also known to use a pair of DC motors to drive the tape spools (as described in FR 2783459, for example). In such a system, both motors operate in a torque control mode to transfer the tape between the spools. A roller which is positioned near to the printhead is used to the determine movement of the tape along the tape path. Such a tape drive requires rollers on the inked side of the tape which can require regular maintenance. Furthermore, desired printing speeds and tape accelerations are increasing, leading to difficulties in successfully operating such a drive.
A motor control system of a tape drive including two brushless DC motors is described in the applicant's United Kingdom patent application number GB1113777.5, also published as US 2013/0039685.